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Comment: A change of direction for solar inverters

12 April 2010Sputnik Engineering AG

The new medium-voltage directive of the German Federal Association of Energy and Water Management (BDEW) demands that photovoltaic inverters no longer turn themselves off after a temporary blackout. Sputnik Engineering AG has adapted their SolarMax products accordingly.

The sweltering heat drives sweat into their faces. Summer is hot like never before. Air conditioners and fans have to work longer and harder than usual. And the power plants reach the limits of their capabilities – the grid is overloaded. Then suddenly the energy supply breaks down. Lifts get stuck, frozen products in supermarkets begin to thaw, the machines in the factory stand still. What sounds like a scene from a horror movie has actually happened in Europe, Asia or America, again and again, in the last few years.

Solar power plants have not been able to help consumers up to now. They have had to shut themselves off during a power outage. In Germany this will now change. The new directive of the Federal Association of Energy and Water Management (BDEW) demands that solar inverters – which feed into the medium-voltage grid – support the grid in such a case from July 2010 on.

Regulating active power dependent on frequency

Since 2009 solar plants in Germany have had to regulate their active power dependent on frequency in order to prevent short-term overloads. SolarMax inverters fulfil these demands when connected to the new data logger MaxWeb xp with the function MaxRemote. From a grid frequency of 50.2 hertz, the inverters automatically reduce the active power fed into the grid and increase it again when the frequency has reached a value of ≤ 50.05 hertz.

The data logger also fulfils the BDEW requirements upon grid security management. “The grid operator provides an active power set-point that the connected plants have to implement within 60 seconds”, explains Michel Ryser, assistant of the development leader at Sputnik Engineering in Biel. At first the utility operator sends a digitally coded signal to a radio ripple control receiver. This then sends, by means of relays, digital signals that the data logger MaxWeb xp interprets and then, in turn, sends to all the connected inverters.

MaxWeb xp: all functions are integrated

“Normally the grid operator signalises four different power levels”, explains Ryser. With reference to the nominal power of the solar plant, they are 100%, 60%, 30% and 0%. At a 100-percent signal, the photovoltaic plant continues to operate as before, at a 0-percent signal, it turns itself off. If, however, commands requiring 30 or 60% are made, MaxRemote reduces the plant output accordingly. The SolarMax inverters do not need an additional interface converter, as many other inverter manufacturers use, for Sputnik has integrated all the functions directly into the data logger MaxWeb xp. This reduces efforts and sinks costs.

The data logger sends the actual feed-in power via the SolarMax web portal or by e-mail to the grid operator. MaxWeb xp saves and logs all processes so that the plant operator can trace the relevant incidents.

Providing reactive power in normal operation

The great change will come in July 2010. Then photovoltaic plants have to remain on grid during short power outages and also provide reactive power in addition to active power. The BDEW directive demands that photovoltaic inverters can be operated at a reactive power that corresponds to a displacement factor of cos phi = 0.95 capacitive up to 0.95 inductive at the grid connection point. The grid operator either gives a fixed or a variable reference value, which has to be attained within a time span of ten seconds up to one minute.

Delivering short-circuit current

In a second step, starting at the beginning of 2011, inverters have to be able to support the grid dynamically by feeding in short-circuit current within 20 milliseconds. In an extreme case, the PV plants have to be able to give off 100% of nominal current as reactive current.

“Solar inverters are ideally suited for the new requirements”, explains Ryser. Technically, Sputnik Engineering has no problems implementing the new regulations. “Sputnik can easily integrate the new functions in the inverter.” SolarMax inverters fulfil the requirements, which have been in force since 2009. In March Sputnik Engineering will present the new inverter series SolarMax TS for the first time in Staffelstein. It fulfils all requirements of the BDEW directive – even those that are required from January 2011 on; it will replace the C series in the summer.

“The new directive shows that grid operators are taking photovoltaic seriously”, says Ryser. “We want to replace conventional power plants with renewable energy sources and drive the expansion of photovoltaic further. Therefore, it’s important that we can also prop up the grid.”

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